Hakan F. Oztop

Estimation of solar radiation using artificial neural networks with different input parameters for Mediterranean region of Anatolia in Turkey

An artificial neural network (ANN) model was used to estimate the solar radiation parameters for seven cities from Mediterranean region of Anatolia in Turkey. As well known that Turkey is a bridge between Asia and Europe and it lies in a sunny belt, between 36^o and 42^oN latitudes. Indeed, the country has sufficient solar radiation intensities for solar applications. In order to make estimation of solar radiation, the data from the Turkish State and Meteorological Service were used. Data of 2006 were used for testing and data of 2005, 2007, and 2008 were estimated. Effects of number of input parameters were tested on solar radiation that was output layer. With this aim, number of input layer parameters changed from 2 to 6. The obtained results indicated that the method could be used by researchers or scientists to design high efficiency solar devices. It was also found that number of input parameters was the most effective parameter on estimation of future data on solar radiation.

Comparison of position of a heated thin plate located in a cavity for natural convection

In this numerical study, natural convection heat transfer in a square cavity with a heated plate built-in vertically and horizontally was investigated. In order to obtain finite difference equations, transport and energy equations were integrated based a finite control volume with non-staggered grid arrangement. The discretized equations were solved with TDMA using SIMPLEM algorithm iteratively. The study was performed for different Rayleigh number values ranging from 10 4 to 10 6 and for different aspect ratios and position of heated plate. Air was chosen as a working fluid (Pr=0.71). The effect of the position and aspect ratio of heated plate on heat transfer and flow were addressed

Natural convection heat transfer by heated partitions within enclosure

Abstract In this study, natural convection heat transfer and fluid flow of two heated partitions. within an enclosure have been analysed numerically. The right side wall and the bottom wall of the enclosure were insulated perfectly while the left side wall and top wall were maintained at the same uniform temperature. The partitions were placed on the bottom of the enclosure and their temperatures were kept higher than the non-isolated walls. The effects of position and heights of the partitions on heat transfer and flow field have been investigated. Computations for Rayleigh number in the range of 104 and 106 have been conducted. Using the control volume approach, finite difference equations are obtained with non-staggered grid arrangement, a computer program based on the SIMPLEM algorithm was developed. The finite difference equations were solved iteratively with a line-by-line Thomas algorithm.

Numerical Analysis of Al2O3/Water Nanofluids Natural Convection in a Wavy Walled Cavity

In the present work, heat transfer enhancement of Al2O3-water nanofluids in natural convection applied to differentially heated wavy cavities is investigated numerically. The governing equations are written in stream function-vorticity form and solved by using the finite volume technique. The effective thermal conductivity and viscosity of the nanofluid are approximated by the Maxwell–Garnetts and Brinkman models, respectively. The solutions are presented by streamlines, isotherms, local and mean Nusselt numbers, and velocity profiles for different Rayleigh numbers (103 ≤ Ra ≤ 105), amplitude of wavy wall (0.85 ≤ a ≤ 1.1), and different volume fraction of nanofluids (ϕ = 0.05 and 0.1). It is observed that the addition of a nanoparticle of Al2O3 into the base fluid increases the mean Nusselt number. It is also more effective on flow field than that of temperature distribution. The geometry parameter or surface waviness can be decided to heat transfer regime even for the same Rayleigh number.

Analysis of Entropy Generation in Natural Convection of Nanofluid inside a Square Cavity Having Hot Solid Block: Tiwari and Das' Model

A computational work has been performed in this study to investigate the effects of solid isothermal partition insertion in a nanofluid filled cavity that is cooled via corner isothermal cooler. Mathematical model formulated in dimensionless primitive variables has been solved by finite volume method. The study is performed for different geometrical ratio of solid inserted block and corner isothermal cooler, Rayleigh number and solid volume fraction parameter of nanoparticles. It is observed that an insertion of nanoparticles leads to enhancement of heat transfer and attenuation of convective flow inside the cavity.

Numerical Study of Entropy Generation due to Coupled Laminar and Turbulent Mixed Convection and Thermal Radiation in an Enclosure Filled with a Semitransparent Medium

The effect of radiation on laminar and turbulent mixed convection heat transfer of a semitransparent medium in a square enclosure was studied numerically using the Finite Volume Method. A structured mesh and the SIMPLE algorithm were utilized to model the governing equations. Turbulence and radiation were modeled with the RNG k-ε model and Discrete Ordinates (DO) model, respectively. For Richardson numbers ranging from 0.1 to 10, simulations were performed for Rayleigh numbers in laminar flow (104) and turbulent flow (108). The model predictions were validated against previous numerical studies and good agreement was observed. The simulated results indicate that for laminar and turbulent motion states, computing the radiation heat transfer significantly enhanced the Nusselt number (Nu) as well as the heat transfer coefficient. Higher Richardson numbers did not noticeably affect the average Nusselt number and corresponding heat transfer rate. Besides, as expected, the heat transfer rate for the turbulent flow regime surpassed that in the laminar regime. The simulations additionally demonstrated that for a constant Richardson number, computing the radiation heat transfer majorly affected the heat transfer structure in the enclosure; however, its impact on the fluid flow structure was negligible.

Estimation of the Mixed Convection Heat Transfer of a Rotating Cylinder in a Vented Cavity Subjected to Nanofluid by Using Generalized Neural Networks

In this study, numerical investigation of mixed convection in a square cavity with ventilation ports filled with nanofluids in the presence of an adiabatic rotating cylinder is conducted. The governing equations are solved with a commercial finite element code (COMSOL). The effects of Grashof number (Gr = 103 to Gr = 105), Reynolds number (Re = 50 to Re = 300), nanoparticle volume fraction (φ = 0 to φ = 0.05), and cylinder rotation angle (Ω = −5 to Ω = 5) on the flow and thermal fields are numerically studied for a range of different parameter sets. The generalized neural network (GRNN) is used to predict the thermal performance of the system. It is observed that the heat transfer increases almost linearly with increasing the nanoparticle volume fraction. The increasing rotation angle in the clockwise direction generally enhances the heat transfer. Moreover, the validation results with artificial neural networks show that generalized neural nets show better performance compared to radial basis and feed-forward networks.

Natural Convection and Entropy Generation in Nanofluid Filled Entrapped Trapezoidal Cavities under the Influence of Magnetic Field

In this article, entropy generation due to natural convection in entrapped trapezoidal cavities filled with nanofluid under the influence of magnetic field was numerically investigated. The upper (lower) enclosure is filled with CuO-water (Al2O3-water) nanofluid. The top and bottom horizontal walls of the trapezoidal enclosures are maintained at constant hot temperature while other inclined walls of the enclosures are at constant cold temperature. Different combinations of Hartmann numbers are imposed on the upper and lower trapezoidal cavities. Numerical simulations are conducted for different values of Rayleigh numbers, Hartmann number and solid volume fraction of the nanofluid by using the finite element method. In the upper and lower trapezoidal cavities magnetic fields with different combinations of Hartmann numbers are imposed. It is observed that the averaged heat transfer reduction with magnetic field is more pronounced at the highest value of the Rayleigh number. When there is no magnetic field in the lower cavity, the averaged Nusselt number enhances as the value of the Hartmann number of the upper cavity increases. The heat transfer enhancement rates with nanofluids which are in the range of 10% and 12% are not affected by the presence of the magnetic field. Second law analysis of the system for various values of Hartmann number and nanoparticle volume fractions of upper and lower trapezoidal domains is performed.

Mhd mixed convection with joule heating effect in a lid-driven cavity with a heated semi-circular source using the finite element technique

A computational fluid dynamics simulation of heat transfer characteristics on the conjugate effect of Joule heating and magnetic field acting normal to the lid-driven cavity with a heated semi-circular source on one wall under constant temperature is investigated. The left wall of the cavity moves in an upward (case I) or downward (case II) direction, and buoyancy forces are also effective. Horizontal walls are adiabatic. The governing mass, momentum, and energy equations along with boundary conditions are expressed in a normalized primitive variables formulation. The finite element method is used in the solution of the normalized governing equations. The study is performed for pertinent parameters such as the Rayleigh number, Hartmann number, and Joule heating parameter. It is found that the average Nusselt number can be decreased with the increasing of the Rayleigh number in the presence of Joule effect. The magnetic field can be a good control parameter for heat transfer and fluid flow.

Conjugate heat transfer in porous triangular enclosures with thick bottom wall

Purpose – The purpose of this paper is to study the conjugate heat transfer via natural convection and conduction in a triangular enclosure filled with a porous medium.Design/methodology/approach – Darcy flow model was used to write governing equations with Boussinesq approximation. The transformed governing equations are solved numerically using a finite difference technique. It is assumed that the enclosure consists of a conducting bottom wall of finite thickness, an adiabatic (insulated) vertical wall and a cooled inclined wall.Findings – Flow patterns, temperature and heat transfer were presented at different dimensionless thickness of the bottom wall, h, from 0.05 to 0.3, different thermal conductivity ratio between solid material and fluid, k, from 0.44 to 283 and Rayleigh numbers, Ra, from 100 to 1000. It is found that both thermal conductivity ratio and thickness of the bottom wall can be used as control parameters for heat transport and flow field.Originality/value – It is believed that this is t...